1. Field of the Invention
The present invention relates to a device for photographing eye movement used for optical viewing in clinical diagnosis to observe, record and analyze an examinee's vertical, horizontal and rotational eye movement using a TV camera.
2. Prior Art
One of the most important diagnostic means used in clinical examination of an examinee's labyrinth, balance of nerves or functional disorders to the central nervous system is the measurement of eye movement. Since examination of a patient's eye movement can reveal symptoms of these problems, various eye movement testing devices are in use.
Commonly used devices such as an ENG, EOG and/or PENG include a polygraph device that records eye movement. Among devices used by the majority of medical practitioners is a device that directly observes eye movement using a 20-dioptry lens equipped with a light source which is called a Fresnel lens. Another device used is a search coil which uses a magnetic field to extract eye movement signals from a coil incorporated into a contact lens which is mounted on the examinee's eye.
However, EOG and PENG devices must be used in a darkened testing room with a separate target display and are problematic in that they cannot examine rotational eye movement. In addition, the Fresnel lens device cannot display targets and cannot record (and therefore analyze) the reactions of the eye movement. Furthermore, the search coil device cannot record a patient's natural eye movement.
In order to solve these problems found in the prior art, Japanese Patent Application No. 63-145425 was proposed by the inventors of the present application. The eye movement testing device of this Japanese application includes a pair of goggles that incorporate visible-ray source targets arranged horizontally and vertically in a cross shape. When used, the eyes are subjected to infrared rays, and the lighting of the targets is switched so that horizontal, vertical and rotational eye movement can be shot in a bright room with a TV camera. The examinee's eye movement image signals are then output from the TV camera for accurate observation, recording and analysis.
In this testing device, a plurality of targets are provided on a reflecting board which is of a prescribed shape so that the targets extend vertically and horizontally from the central target which is positioned on the axis of the eye. The optical axis of the TV camera is located proximately to the central target and penetrates the reflecting board. Accordingly, the image of the eye (the anterior part of the eye) on the light receiving element surface of the TV camera shows deviations both vertically and horizontally off the center of the pupil. Thus, measurement tends to be inaccurate.
Specifically as shown in FIG. 4, when the axis running through the center of the pupil of the anterior part C of the eye (which includes the cornea of the eyeball E) moves for the same angle in a direction A or B from the eye axis L-1, the eye axis L-1 runs through the central target of the cross-shaped target groups. However, the TV camera's optical axis L-2 running through the center of the pupil deviates for an angle .alpha. from the eye axis L-1 (shown on a single plane for the sake of explanation). Thus, though the image is formed on the image-forming surface 2a' of the light receiving element 2a positioned behind the shooting lens L and by the spherical-shaped anterior part of the eye Ca (with a circular front) shifted in the direction A from the eye axis L-1, such an image will be elliptical in shape and of a narrow width. If the image formed on the anterior part of the eye Cb, which is shifted in the direction B, the image will be elliptical in shape (Ib) and of a wider width, thus causing a difference in the widths of the images.
As can be seen from the above, even when the eye axis is moved (or shifted) an equal amount (distance) in either the A or B direction, deviation in the A direction will be smaller than deviation in the B direction on the image-forming surface of the light receiving element. However, since the resolution on the light receiving surface is constant at any position when the eye axis is shifted in the direction A (i.e., rotated away from the TV camera's optical axis), resolution deteriorates and measurement accuracy becomes lower compared to when the eye axis is shifted in the direction B.